The applicant has shown in U.S. Pat. Nos. 5,004,574 and 5,405,668 various structural shapes of plastic rods and tubes which are reinforced in strength by the presence of fibers or filaments running lengthwise and being encased in the plastic. Methods for making such items are also disclosed in these patents. Among the uses for such technology is the making of poles to replace wooden poles that are getting more costly as less wood is available for such uses. Wooden poles are subject to deterioration from the atmosphere and from being buried in the soil. Plastic materials can readily replace wooden poles in many, if not all uses, and it accordingly is the primary purpose of this invention to provide a method for preparing poles, beams or other structural load bearing elements from plastic materials. In order to reduce weight and cost it is preferred that such poles, beams or elements be made hollow; and in order to provide sufficient strength for minimum volumes, it is preferred that the plastic material be reinforced by fibers or filaments generally running lengthwise of the element so as to provide flexural strength. This invention is useful for structural cantilever members, e.g., in airplane wings, spars or the like; masts or poles of any size or shape, e.g., power poles for high tension electricity, poles for lights, fence poles and poles and beams for all sorts of building or construction supports.